: The concept of a "particle" becomes local and observer-dependent. Different observers (e.g., one inertial and one accelerating) may disagree on whether a state contains particles or is a vacuum.
Quantum Field Theory in Curved Spacetime (QFTCS) is a "hybrid" framework where matter fields are treated according to quantum principles, but the gravitational background is described classically by a fixed spacetime metric gμνg sub mu nu end-sub Quantum Field Theory in Curved Spacetime: Quant...
: In the early, rapidly expanding universe, time-varying gravitational fields can "excite" the vacuum, creating elementary particles that seed the large-scale structure of the universe. Robert Wald - Quantum Field Theory in Curved Spacetime : The concept of a "particle" becomes local
: A second observer might decompose the same field using a different basis and operators Mixing : The new annihilation operator Robert Wald - Quantum Field Theory in Curved